A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    ⋆                      λ                          NA              PS                                                          (        1        )            where λ is the wavelength of the radiation used, NAPS is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NAPS or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. An EUV radiation source may be configured to output radiation having a wavelength for example within the range of 10-20 nm. Thus, EUV radiation sources may constitute a significant step toward achieving a reduction of the critical dimension which may be achieved using lithographic apparatus. Such radiation is termed extreme ultraviolet or soft x-ray, and possible sources include, for example, laser produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings.
In a laser produced plasma source, a laser beam is directed onto droplets of fuel, thereby causing the fuel to vaporize and form a plasma. The plasma emits EUV radiation which is collected by a collector (often a curved mirror) and focused to a focal point. In some instances, vaporization of the droplets of fuel may be incomplete. As a result of this incomplete vaporization, debris is introduced into the source and may accumulate on the collector. In addition, vaporized fuel (which may also be considered to be debris) may remain in the source and may also accumulate on the collector. The accumulation of debris on the collector causes the collector to lose reflectivity over a period of time. As a consequence of this, it may be necessary to periodically remove and replace or clean the collector. Operation of the lithographic apparatus is suspended while the collector is being replaced or cleaned, causing an interruption in the patterning of substrates.
It is desirable to reduce the incidence of debris on the collector of a laser produced plasma source, since this will extend the intervals between replacement or cleaning of the collector.